The present invention relates to a temperature detecting transistor circuit utilizing a temperature characteristic of transistors.
Using a thermal breaker circuit to forcibly turn off an output transistor acting as a heat source when the temperature of the substrate of a semiconductor integrated circuit (IC) rises above a predetermined value in order to prevent the IC from being thermally damaged, is well known. A prior art thermal breaker circuit utilizes the temperature characteristic of a transistor breaker as shown in FIG. 1. In FIG. 1, a series circuit is shown including a constant current source 1 and a Zener diode 2 connected with a backward-bias between a power source terminal V.sub.cc and a ground terminal G. Another series circuit including resistors 3 and 4 is connected across the Zener diode 2. A transistor 5 for temperature detection is connected at the base electrode to a node B connecting the resistors 3 and 4, at the emitter to the ground terminal G, and at the collector to an output terminal OUT.
In FIG. 1, assuming that a voltage across the Zener diode 2 is V.sub.Z, resistances of the resistors 3 and 4 are R.sub.3 and R.sub.4, respectively, and a voltage across the resistor 4 is V.sub.4, the following equation (1) holds ##EQU1## When the voltage V.sub.4 exceeds a base-emitter voltage V.sub.BE5 of the transistor 5, a collector current (or output current) I.sub.OUT flows through the collector of the transistor 5.
In FIG. 2, where the abscissa indicates an absolute temperature T.degree.C. and the ordinate the voltages V.sub.BE5 and V.sub.4, a rectilinear line a indicates a temperature characteristic of an ordinary transistor (a change of the base-emitter voltage V.sub.BE5 in relation to temperature T.degree.C.). Another rectilinear line b is representative of a temperature characteristic of the Zener diode 2 (a change of the voltage V.sub.4 in relation to temperature T.degree.C.). As seen from this graph, the Zener diode 2 has a positive temperature coefficient (See the line b), while the base-emitter voltage V.sub.BE5 of the transistor 5 has a negative temperature coefficient (See the line a). Further, this graph indicates that, when temperature exceeds a temperature point T.sub.0, the transistor 5 turns on to allow the collector current I.sub.OUT to flow. If an output transistor (not shown) acting as a heat source is connected to the output terminal OUT, the output transistor can be cut off by the use of the output current I.sub.OUT. Thus, the thermal breaker circuit can protect the IC from being thermally damaged.
To operate the circuit shown in FIG. 1, a voltage V.sub.cc applied to the power source terminal V.sub.cc is required to be higher than the Zener voltage V.sub.z (about 6 V) of the Zener diode 2. However, this requirement is undesirable in IC's, because the power source voltage Vcc for the IC should be decreased in order to decrease the power consumption. Further, the temperature coefficient of the Zener voltage V.sub.z is only 0.07%/.degree.C., so that the temperature sensitivity of the overall temperature detecting circuit is low.